Question

Calculate the number of molecules present in 7.2 gms of pentane gas. $\left(C_5{H}_{12}\right)$

• A. Number of molecules $=60\times {10}^{21}$
• B. Number of molecules $=6\times {1000}^{23}$
• C. Number of molecules $=60\times {10}^{23}$
• D. Number of molecules $=6\times {100}^{24}$

Answer 1

The correct option is A

Number of molecules $=60\times {10}^{21}$

Molecular Mass of pentane $C_5{H}_{12}$ = $\text{5}\times \text{12}+\text{12}\times \text{1}$

= 60 + 12 = 72g

Number of moles of pentane $=\left(C_5{H}_{12}\right)$

$=\frac{\text{Mass}}{\text{Molecular Mass}}=\frac{7.2}{72}$

= 0.1
1 mole of pentane contains $6.022\times {10}^{23}molecules.$

Number of molecules in 0.1 mole pentane

$=0.1\times 6.022\times {10}^{23}$

$=6.022\times {10}^{22}$ molecules

$=60.22\times {10}^{21}$ molecules